Personal checks used to move cross-country on planes and trains, but a new law going into effect Thursday is putting processing on the electronic superhighway — possibly at the expense of consumers.

The law, called Check 21 (search), promises to save the banking industry $2 billion per year by electronically processing checks, which will eliminate the physical transportation involved now.

"The purpose of Check 21 was to make the check processing system more efficient. It's pretty antiquated at this point," said Nessa Feddis of the American Bankers Association (search), who also said the change will be gradual, not sudden.

But consumer watchdogs say the instantaneous clearing of checks will cost check-writers since banks will still be permitted to hold deposits as long as they do now.

"There's no requirement that deposits be made available any sooner, so money could be leaving your account more quickly than it's coming in," said Gail Hillebrand, a senior attorney at the Consumers Union (search).

Some groups are warning people who rely on a "float," which is when a check is written knowing it won't clear until after funds have been deposited.

"If you walk in somewhere and you're going to write a check ... that's a little bit frightening if you know the next day you might have to cover it," said Gus Koustis, who occasionally "floats" a check.

Soon, legally binding photocopies will replace cancelled checks in your monthly statement.

Click in the box near the top of this story to watch a report by FOX News' Jeff Goldblatt.